It is well known that in the common power devices, the reverse voltage applied across the n+ region(s) and the p+ region(s) is sustained by a lightly doped semiconductor layer, called hereafter a Voltage Sustaining Layer. For high voltage power devices, the on-resistance Ron (or the on-voltage) is also dominantly determined by the voltage sustaining layer. The lower the doping concentration is and/or the larger the thickness is, the higher the breakdown voltage is, but the larger the on-resistance (or the on-voltage) also is. For many power devices, one of the most important problems is to get both high breakdown voltage and low on-resistance. The relation between them has become an obstacle for manufacturing high performance power devices. Moreover, said Ron refers to the conduction area of the voltage sustaining layer, but practically there always exist some areas which do not contribute much to the conduction. For example, the area under the source body in a vertical MOSFET (metal oxide semiconductor field effect transistor) and the area under the base contact of the bipolar transistors are both the areas which do not contribute much to the conduction.
The inventor's CN patent ZL91101845.X and U.S. Pat. No. 5,216,275 have solved the problems above, by using a composite buffer layer (CB-layer) between the p+ region(s) and the n+ region(s) to sustain voltage. The CB-layer contains two kinds of regions with opposite types of conduction. The two kinds of regions are alternately arranged, viewed from any cross-section parallel to the interface between the CB-layer and the n+ layer (or the p+ layer). However, the hitherto-used voltage sustaining layers are the semiconductors of a single conductivity type. The above inventions also disclose a MOS tube with such voltage sustaining layer, of which the on-resistance Ron in unit area is proportional to 1.3rd power of the breakdown voltage VB. This represents a breakthrough for the relation of the common voltage sustaining layers; meanwhile, the other electronic performance of the MOS tube also acts well.
In the past few years, great changes have taken place in the semiconductor power devices industry. The MOS tubes having the super-junction device structure (that is CB-layer structure) can provide high voltages and large currents.
FIG. 1(a) to FIG. 1(b) show a method for manufacturing a super-junction power device 1. The process begins with a wafer of substrate 2 on which a first epitaxial layer 3 grows. The substrate 2 in these figures is a heavily doped n+ layer, while the first epitaxial layer 3 is a lightly doped n layer, in which the ions are implanted to form a layer of the p type region 4. Generally speaking, one epitaxial layer is required to sustain every 50 to 100 voltage. Therefore, for a 600V transistor, it is required to form n type epitaxial layers 5,7,9,11 and 13 shown in FIG. 1(a); after each epitaxy process, it is required to form p type ion-implant layers 6,8,10,12 and 14 shown in FIG. 1(a).
After the diffusion of the p type ion-implant layers 4,6,8,10,12 and 14, p region 16 is formed, as shown in FIG. 1(b). The region without influence of ion-implantation is n region 15. Thus the alternately arranged p region and n region are created. Thereafter, a device layer or so-called device feature layer 17 is manufactured. The device feature layer 17 contains the ion-implanted n+ source regions 18, the oxide layers 19 and the metal gate or polycrystalline silicon gate 20 thereon. A p+ region 21 is provided between the two n+ source regions 18. A deep junction p+ region 22 is under the p+ region 21. The deep p+ region 22 connects to the p+ region 21.
Obviously, the above methods are very costly. Moreover, as some defects are introduced in each epitaxy process, the more time the epitaxy process is developed, the lower the quality of semiconductor is and the lower the quality of device is. Besides, the CN patent ZL91101845.X and U.S. Pat. No. 5,216,275 describes another important case that a thin dielectric layer is between the alternately arranged n region and p region. The above method apparently can't be used in such condition.